The present invention relates to a sealing structure of a fuel cell, particularly of a solid polymer-type fuel cell; a fuel cell separator unit useful in realizing this sealing structure; and a process for producing the same (or a molding process of a rubber packing used in the sealing structure).
A solid polymer-type fuel cell comprised of a cathode electrode, an anode electrode, and such a polymer electrolytic membrane as a membrane of an ion exchange resin having ion conductivity interposed therebetween starts generating electricity by, for example, supplying a fuel gas such as hydrogen gas to the anode electrode and an oxidant gas such as an oxygen gas or air to the cathode electrode thereby to cause an electrochemical reaction and convert the chemical energy of the fuel gas into electric energy (electricity).
Such solid polymer-type fuel cell is usually constituted of a fuel cell assembly composed of a plurality of stacked unit cells. Between adjacent single cells are provided separators. Each separator is provided with fuel gas passageways and oxygen gas passageways, with which the fuel gas and the oxidant gas are isolated. Each electrode and its adjoining separator need to be gas-sealed tightly thereby to prevent the leakage of fuel gas or oxidant gas from the edge or periphery of the polymer electrolytic membrane. Therefore, in assembling a fuel cell, a sealing structure is usually constructed by having a rubber packing (particularly, a thin rubber packing) made by compression molding, injection molding, or by punching out of a sheet be present between an electrodes and a separator.
In the sealing structure described above, however, the gas sealing against the fuel gas and the oxidant gas needs to be strictly retained for a long period of time, leading to a need for the rubber packing to be improved in its integrity and durability. Since the above-described rubber packing is a very thin membrane, forming such flexible packings by compression molding or injection molding not only results in variations in film thickness consequently leading to a lack in integrity but also makes their positioning at predetermined locations of the fuel cell difficult. Moreover, when assembling a fuel cell, the rubber packings are sometimes deformed or dislodged and therefore unable to provide a sure sealing.
U.S. Pat. No. 5,176,966 discloses the fabrication of fuel cells, in which a solid polymer ion exchange membrane and carbon fiber paper layers between which the membrane is interposed are heat-pressed for unitarily joining them into a single assembly, and the single assembly is processed to provide the assembly with sealing grooves. Then, a sealant of silicon rubber is injected into the grooves for sealing, and a fuel cell is assembled by using guide pin holes. The fabrication of this fuel cell, however, involves providing the carbon fiber paper layers integrally joined to the ion exchange membrane and constituting the unit assembly with sealing grooves. In addition to having the possibility of giving damage to the carbon fiber paper layers, this is not easy, much less the injection of the sealant into the grooves.
Japanese Patent Application Laid-Open No. 92450/1998 (JP-A-10-92450) discloses a sealing material for sealing a cell and separators constituting a solid electrolytic fuel cell, in which the sealing material is a glass material which softens at a temperature lower than the operating temperature of the solid electrolytic fuel cell and crystallizes at the operating temperature to be solid crystalline glass. Such sealing material, however, needs to be set at a predetermined place between the cell and the separators with accurate positioning. As a result, an easy and efficient assemblage of the fuel cell cannot be achieved.
Thus, an object of the present invention is to provide a sealing structure which realizes a sure placement of a rubber packing at a predetermined position and ensures a tight sealing, a fuel cell separator unit useful in constructing this sealing system, and a process for producing the same (or a molding process of rubber packings used in the sealing system).
Another object of the present invention is to provide a fuel cell separator unit on which a rubber packing of high integrity and durability can be formed with high working efficiency even if the packing is thin, and a process for producing the same (or a process of molding a rubber packing used in the sealing system).
Another object of the present invention is to provide a sealing structure for a fuel cell which realizes the provision of a sure sealing and the assemblage of a fuel cell assembly with high productivity as well as the omission of a rubber packing placement step, a fuel cell separator unit useful in constructing this sealing system, and a process for producing the same.
The inventors of the present invention found that forming a vulcanized or crosslinked rubber layer as a rubber packing on the periphery or edge of a separator of a fuel cell realizes a sure placement of even a thin rubber layer packing at a predetermined location, ensuring a tight sealing, which eliminates the need for a step of attaching the rubber packing between the electrode and a separator. The present invention was accomplished based on the above findings.
That is, the sealing structure of a fuel cell of the present invention is comprised of a single cell, a separator on each side of the single cell, and a packing interposed between the single cell and the separator. The packing is constituted of a vulcanized or crosslinked rubber layer and adhered to the periphery of the separator. The single cell comprises an electrolytic layer (e.g., solid polymer electrolytic membrane), and a cathode electrode and an anode electrode provided on both sides of the electrolytic layer.
The fuel cell separator unit of the present invention is a separator unit provided on each side of the single cell of the fuel cell and capable of sealing the periphery or edge of the single cell. The separator unit comprises a gas-impervious separator and a vulcanized or crosslinked rubber layer formed on the periphery of the separator. The rubber layer can be vulcanized or crosslinked by using a vulcanizing agent or a crosslinking agent, or by irradiation of radioactive rays.
The fuel cell separator unit can be fabricated by coating a rubber-containing coating agent on the periphery of the separator and then vulcanizing or crosslinking the rubber layer. The coating fluid can be screen-printed on the periphery of the separator (e.g., in the form of a loop), and a thin rubber layer integrally joined to the separator can be formed by directly applying the coating agent on the surface of the separator, drying the coat, and then vulcanizing or crosslinking the rubber.